I just got through changing about 80% of my tank water out after it had been cloudy for a few weeks, I had been doing normal 30-50% changes but had not seen any results in the way of removing the cloudiness. I stupidly forgot that the tap water should be at 76F before adding it to the tank. Therefore when I woke up this morning all of my electric yellow labidos(3) and my bala sharks(2) were dead although they looked fine last night after the change. My angelfish and redfinned shark and pleco all survived and are doing fine but I guess my tank heaters didn't have enough time to adjust the tank temperature. I just wanted to post this to warn other new tank owners to not make the mistake I have made. I have had my 55gallon tank since december and this site has helped me greatly. But only one person can stop stupidity and that is YOU! so be ware._________________Just because it hasn't been done doesn't mean it can't be done.

I would not medicate already stressed fish until I saw disease, personally.
I think meds are as hard on the fish sometimes as the illness is, so I prefer not to add medications unless I'm 100% sure they are necessary.

Sherry... IMO if you use a product such as Aquarisol it will aid the fish. It's more of a preventitive, but will get rid of ich in a timely fashion. I think if he waits to see the ramifications from his 80% WC he'll have fish that may be too far along to help. He didn't indicate what temperature the water was that he added but by his post I'd say it wasn't warm(70-80 degree) water.

thnks for the warning but I already have an ich medication here by the tank but as yet have not seen any indication of ich, although the angel fish has a slight case of fin rot that seems to already be going away. _________________Just because it hasn't been done doesn't mean it can't be done.

fish keeping is all about learning from mistakes and not repeating them. Your story goes along way to educating and its good to share.
I would like to hypothesize on the cause of death here. I have seem temperature shock, both cold and heat. Your story doesn't sound like any temp shock I have seen. I have also seen 80% tap added to Bala's before resulting in 100% fish kill.
I believe that the labs and Bala died due to chloramine or a heavy metal contaminant in the water. When doing massive water changes with tap water I always use the dose appropriate for chloramine as opposed to just chlorine.
Still the cold water was definately a factor any way you look at it._________________"Just don't look in my fish room honey... it's just better for all of us. "

My local water supply says they don't use chlorine or chloromine, instead they use uv lighting. But I err on side of caution and always dose for chlorine anyways just to be safe. I had dosed for chlorine this time so I don't know if chloromine would have been an issue or not but chlorine shouldn't have been in the water at all._________________Just because it hasn't been done doesn't mean it can't be done.

I don't know if you can blame an 80 % water change alone for the deaths. A few times, mostly from moving a tank, I have drained at least 75% of tank water (left fish and everything in the bottom of the tank) and then when arriving in thenew location filled with new chlorine / chloramine remover treated water and I have never lost a fish. While you drained the water, could your tank have developed depleted oxygen levels?

The problem was not just the water change it was that I did not heat the water I put in so it was about 50 degrees F and the tank was 76F that is what happened. They died from temp shock, reread the original post it explains it all there my friend.